1. Field of the Invention
The present invention relates to focusing electrodes in an electron gun for use in a color television cathode ray tube and, more particularly, to focusing electrodes, wherein a third focusing electrode is united with a long extension member to which the third focusing electrode is electrically connected and a fourth focusing electrode is united with a short extension member to which the fourth focusing electrode is electrically connected.
2. Description of the Prior Art
A color television has an electron gun comprising cathodes generating three red, green and blue electron beams and various grid electrodes accelerating and focusing these beams. The electron gun may be of the delta configuration or in-line type, both of which produce three focused electron beams, form spots on a screen and further, reproduce an image.
It is generally known that main elements of the electron gun which form a spot are a third electrode and a fourth electrode. When these electrodes are energized at different voltages, these electrodes form a main electron lens in themselves and thereby, cause electron beams to be focused on the multiple phosphor groups located on the faceplate of a color cathode ray tube.
FIG. 5 is a sectional view of the third electrode and the fourth electrode in a general electron gun. The third electrode G3 and the fourth electrode G4 have three apertures a1, a2 and a3 and b1, b2 and b3, respectively, as beam paths. The middle aperture b2 is aligned with the adjacent middle aperture a2 to provide a substantially symmetrical beam focusing electric field between apertures a2 and b2 when electrodes G3 and G4 are energized with different voltages. The two outer apertures b1 and b3 are slightly offset outwardly with respect to the corresponding outer apertures a1 and a3, to provide an asymmetrical electric field between each pair of outer apertures when electrodes G3 and G4 are energized, to individually focus each outer beam near the screen, and also to deflect each outer beam toward the middle beam to a common point of convergence with the middle beam near the screen.
With respect to an electron gun of in-line type, a convergence angle is obtained from the following general expression: EQU .theta.=K.multidot.(d/D).multidot.(Eb/Vf)
.theta.: Convergence angle PA1 K : Constant PA1 D: Diameter of beam paths in a third electrode PA1 d: Asymmetrical distance PA1 Vf: Voltage in a third electrode PA1 Eb: Voltage in a fourth electrode.
It is known from the above general expression that the convergence angle .theta. is proportional to the voltage ratio of main electron lens Eb/Vf and the asymmetrical distance d.
The aforementioned in-line electron gun has structural deficiencies in that it is inevitable that it will generate a difference in focus voltages in three guns and cause convergence error and thereby, the picture quality on the periphery of the screen becomes indistinct.
The difference of focus voltages occurs for the following reasons. Firstly, a local voltage difference of 100-200 Volts is generated by the diameter difference between two outer apertures b1 and b3 and the middle aperture b2 in the fourth electrode G4. Secondly, focus is affected by the high potential field in the neck region encompassing the electron gun. The deteriorated picture occurs on the peripheral part of the screen, because beam spots shown in FIG. 6 become elliptical on the peripheral part of the screen by means of astigmatism occurring at the time of deflecting beams.
There has been provided an electron gun which removes the above disadvantages by means of enlarging the main focusing lens and thereby, decreasing the effect on the focus voltage difference.
One example is an "embodiment of in-line electron gun" described as in 79-49862 in the Japanese Utility Model Gazette for laying-open of unexamined applications. With respect to said electron gun, the opposite faces of the third and fourth electrodes are formed to be deepdrawn inwardly so that the distance between each three apertures of the third and fourth electrodes becomes more distant and thereby, it is tried to decrease spherical aberration. But this electron gun cannot obtain the desired result because mutual interference is deeply generated among the potential distribution of electron beams R, G and B forming between the third and fourth electrodes.
As another prior art for eliminating mutual interference of the potential distribution, there is a "unitized electron gun having electrodes with internal beam shielding tubes" described and claimed in the U.S. Pat. No. 4,119,884. This electron gun is of the so-called discrete type in that there electron guns are positioned separately. It is able to eliminate mutual interference of the potential distribution of electron beams R, G and B, but incapable of improving the effect on the high potential received from the neck region and the deterioration of picture quality forming on the peripheral part of the screen due to astigmatism.